Disk drives are utilized in many information and processing systems for storing data. A typical disk drive includes a spindle motor for rotating a data disk. The spindle motor can be a brush less DC motor having multiple phase windings arranged as a stator, and a rotor having a permanent magnet for rotating the disk. The motor is commutated to start from standstill and maintain an operational speed by sequentially energizing appropriate phase windings based on the location of the rotor to the phase windings. The energized windings generate torque inducing magnetic fields relative to the rotor magnet that rotate the rotor.
One method of energizing the windings is via Pulse Width Modulation (PWM) to conserve power. In PWM, a train of pulses are applied to energize the windings, whereby power to each winding is switched on and off. Since each winding is an inductive load, a change in current flowing through the inductive load during switching causes the voltage across the load to rise or fall as a ramp. The power dissipated by the load during switching is a function of the duration and amount of current flowing through the load during switching. Therefore, controlling the slew-rate of the ramp voltage across the inductive load can reduce power dissipation by the inductive load in PWM. Load voltage slew-rate control also helps reduce electromagnetic disturbance and coupling to other circuits. Further, slew-rate control reduces acoustic noise generated by the inductive loads, when switching the loads on and off, by smoothing out any spikes in the ramp voltage across the load.
Existing slew-rate controllers typically include a field effect transistor electrically connected to an inductive load at the drain of the transistor to control the rate of change of voltage across the load at turn off. The gate-drain capacitance of field effect transistors causes a feed back current to the gate of the transistor (Miller effect) leading to instability and oscillation. To alleviate this problem, conventional controllers utilize a capacitor with one end connected to the drain of the field effect transistor and another end connected to the input of a voltage amplifier, and the output of the amplifier is connected to the gate of the transistor. A change in the load voltage at the drain of the field effect transistor causes a current to flow through the capacitor and change the voltage at the input of the amplifier. The voltage amplifier amplifies the input voltage and applies an amplified voltage change to the gate of the transistor, thereby controlling the load voltage slew-rate at the drain.
One disadvantage of such controllers is that to handle large load voltage ramp slopes, the capacitor must have high capacity. High capacitance requires calls for sizeable capacitors, occupying precious physical space in integrated circuits. And, embedding large capacitors in such circuits is expensive. Another disadvantage of such controllers is that the voltage amplifier has a high voltage gain and can over-correct the voltage at the gate of the field effect transistor. A minor change in the voltage at the input of the amplifier causes a major change in the voltage at the gate of the field effect transistor, leading to ringing and control instability at the drain of the transistor. To compensate for the instability, a larger capacitor must be used, occupying even more precious space. Yet another disadvantage of such controllers is that the high impedance at the input of the voltage amplifier causes noise as the voltage amplifier switches operating modes in response to voltage changes at its input.
There is, therefore, a need for a voltage slew-rate controller with reduced circuit dimensions which provides substantially stable slew-rate control with reduced noise and ringing.